The relative likelihood of adaptation to a variable environment by genetic divergence and phenotypic plasticity will be studied. The response of thorax size to temperature in Drosophila melanogaster will be used as a model system. The responses to two types of selection, directional selection on thorax size and selection on plasticity of thorax size, will be contrasted. The response to directional selection on thorax size indicates the potential for adaptation by genetic divergence. The response to selection on plasticity of thorax size indicates the potential for adaptation by phenotypic plasticity. The relative rates of response and the amount of change in heritable variation of thorax size and plasticity of thorax size measures the relative likelihood of these two modes of adaptation to a variable environment. These potentials will be assessed by measuring a stock population then performing twenty generations of each type of selection and measuring the resultant changes in the selected lines. The existence of different allelic variants for thorax size and plasticity of thorax size will be substantiated by extracting single chromosomes from each of the selected lines and testing them against a standard background genome. Changes in developmental homeostasis and the plasticity of other traits will also be measured. The results of this study will indicate how theoretical models of adaptation to variable environments need to be reconsidered in the light of developmental constraints.